1. Field of the Invention
The present invention relates to a method of decoding time data of a video image in a video decoding apparatus.
2. Description of the Related Art
In MPEG-1, MPEG-2, H. 261, and H. 263 encoding systems that are international standards of video encoding, only one item of information containing time data for a variety of time controls during decoding exists in one frame. In contrast, MPEG-4 that is an international standard for encoding a video image is a system having error resilience, assuming use in a transmission system with a plenty of errors such as radio communication including mobile communication. With respect to frame time data as well, error resilience is ensured to a certain extent.
That is, in the MPEG-4, sync restoration markers (resync markers) are inserted into one frame, and each frame is divided into a plurality of packets, making it possible for such each packet to provide header information containing time data. In a video image encoding system such as MPEG-1 and MPEG-2 in which time data exists in only the first header information in one frame, time data could not be decoded if the first header information is not correctly decoded due to an error or the like at the decoding side. However, if a plurality of header information items containing time data are provided in each frame, even if an error occurs, frame time data can be always decoded.
Thus, a system in which a plurality of header information items containing time data in each frame are provided includes a problem that time data of the header information items in the same frame indicate different values due to an error. In such a case, it is not specified conventionally as to what item of time data is employed as frame time data.
In Jpn. Pat. Appln. KOKAI Publication No. 10-234033, there is disclosed a system for determining an error according to whether time data is present or absent within the range of an frame-interval time as follows. That is, a counter for counting the number of bits in an inputted encoded bit stream is provided in order to determine whether or not decoded time data contains an error. The number of bits in the bit stream of the received previous frame is counted, the number of generated bits is obtained, and an frame-interval time is obtained by the number of generated bits and the control threshold and transmission speed of a transmission buffer. Then, this frame-interval time is compared with decoded time data.
This system is applicable even if a plurality of header information items containing time data exist in one frame. However, determination processing, in particular, processing for obtaining an frame-interval time using a number of parameters such as the number of bits in an encoded bit stream or the number of generated bits is very complicate. Thus, there is a problem that this system is disadvantage in view of cost efficiency or requires a large amount of processing time.
As described above, in a video image decoding apparatus, it is required to decode correct time data irrespective of an error. However, there has been a problem that a system of determining an error of time data based on a conventional technique is very complicate, which requires a large amount of processing time.